The maintenance of aging steel structures, such as bridges and overpasses, poses a serious problem for governmental officials. These structures usually have layers of paint to prevent corrosion which, in certain cases, must be removed before reconditioning with new paint. The problem is that the old paint, especially the primer undercoat paint layer, may contain lead. The removal of this lead-based paint is typically accomplished by a sandblasting technique. However, the waste created by this process is a large volume of waste sand contaminated with metals, such as lead. The contaminated sand must be disposed of as a toxic material under environmental regulations, and this disposal is therefore expensive. For example, the reconditioning of a typical highway bridge can generate up to 800 tons of waste sand which could cost up to about $400,000 to dispose in a landfill.
There exist various methods for the treatment of contaminated waste soils, such as those process systems shown in U.S. Pat. Nos. 5,128,068; 5,056,541; 4,841,998; and 4,783,263. These processes typically rely upon size and density separatory techniques to purify the waste soil. The use of these techniques does not enable production of a cleaned sandblasting sand to environmentally acceptable levels, because the paint chips are not easily separated from the sand on the basis of size or density.
A need therefore exists to provide a process for the treatment of waste sandblasting sand that contains paint chip particles with metal contaminants. Further, the process must be economically competitive with other waste disposal means and it must be practical in design so that it can be practiced on location at decontamination sites.